The midfielder arrives from Dinamo Zagreb.
Moratti told inter.it: "We've gone for players that we think suit the way we play.
"I think Kovacic is a particularly good signing - I think that he's a special player. He's very talented and very young, so he fits in with our plans perfectly.
"He's a special talent and he should be interesting to watch because I'm sure he'll give us something new and that's always nice.
"Football is beautiful when you see something you've never seen before."